Venus, otherwise known as the "Evening Star" and the "Morning Star," is the second planet from the sun, the closest planet to Earth in size, and the planet that approaches closer to Earth than any other planet.

Ancient knowledge and naming

Venus has been known to man for all of recorded history.[7] The ancients originally misidentified the "Evening Star" and the "Morning Star" as two separate objects.[3][7] Tradition credits Pythagoras with realizing that evening and morning star were the same object.

Venus is the Roman name of the goddess of love and beauty. Venus is identified with Astarte,[8] or Ishtar, chief goddess of Babylon. Venus was also known to the ancient civilizations of the Americas; the Aztecs called it Tlahuizcalpantecuhtli, or Quetzalcoatl,[9] and the Mayans Kukulcan.[10] In addition, all the geographical features of Venus are named for goddesses from one body of mythology or another.

Venus is important historically as a calendar reference, and is still of high standing in astrology. More to the point, Claudius Ptolemy considered that Venus orbited the earth, in an orbital sphere one level closer to Earth than the sun, but subject to a wide epicycle.

Orbital characteristics

Venus' orbit is more nearly circular than that of any other planet in the solar system.[7] It maintains an average distance of 108,210,000 km from the sun. Its closeness to both the earth and the sun, and its very high geometric albedo (65%), make it the brightest object in the night sky except for the Moon.

The sidereal period of Venus is 224.7 days, and the synodic period is 583.92 days. Remarkably, the Mayans calculated Venus' synodic period at 584 days, a figure accurate to within an incredible three significant digits.[10] Moreover, the ratio of the synodic year of Venus to the year of Earth is very nearly 13:8, close enough to suggest that Earth and Venus participate in an orbital resonance.

Venus's orbit is inclined 3.39 degrees from the ecliptic. For this reason, Venus usually passes north or south of the sun in its conjunctions with earth. However, every century or so, Venus makes a pair of transits across the Sun, eight years apart. Indeed, the transit of Venus in 1769 allowed astronomers to determine the length of the astronomical unit.[11]

Venus made such a transit on June 8, 2004, and will make another on June 6, 2012. After that, astronomers do not expect another transit until the year 2117.[12]

Phases of Venus

Like the Moon, Venus does show phases. Galileo Galilei was the first to observe the phases of Venus. If Venus's motion were confined to a Ptolemaic epicycle in an orbit between the Moon and the Sun, Venus would never show any phase but a crescent—but Galileo observed a full range of phases, from waxing crescent to "first quarter" to waxing gibbous to full to waning gibbous to "last quarter" to waning crescent to "new." Such phases proved inexplicable by the Ptolemaic model and constituted the strongest evidence in favor of Nicolaus Copernicus's model of heliocentricity.[13]

Venus's waxing phases are on its left side, whereas the Moon's waxing phases are on its right side. This shows that Venus moves about the Sun in the same direction that the Moon moves around the earth.

Rotational characteristics

Venus rotates retrograde, or east to west, with a sidereal day of 5832.5 hours, or 243.686 Earth sidereal days. Venus' solar day is considerably shorter, at 2802.0 hours (117 sidereal days). This means that on Venus, it's day is longer than its year.Oddly enough, the solar day of Venus is almost exactly one-fifth its synodic year with respect to the earth, with the result that at each inferior conjunction (closest approach) of Earth and Venus, the same (night) side of Venus faces the Earth. This has led some astronomers to speculate that Earth and Venus are in some kind of tidal lock. However, at least two authorities stated that and Earth-Venus gravitational interaction would not produce such a lock unless the sun itself were producing a "tide" in Venus's atmosphere.[14]

Physical characteristics

Venus's mass is 4.8685 * 1024 kg, or about 81.5% that of earth.[1] Its radius is 6051.8 km. Perhaps on account of Venus' very long sidereal day, Venus is an almost perfect sphere.

Atmosphere

Venus has the most dense atmosphere of all the terrestrial planets, and the most poisonous. Its pressure is 92 times the average sea-level pressure on Earth.[1] The most prominent gases in the atmosphere is carbon dioxide (96.5%) and nitrogen (3.5%). But Venus also has 150 ppm of sulfur dioxide, which forms clouds that permanently obscure the view of Venus' surface from space.

The surface of Venus is hotter even than the surface of Mercury. Conventional theories say that a runaway greenhouse effect is responsible. Recently, however, a new "moist greenhouse" model has been proposed. One reason for this proposal is to explain the disappearance of the vast quantities of water that, according to conventional models, Venus must have begun with.[2] According to the "moist greenhouse" model, the water boiled off over a period of 600 million years, beginning 4 million years ago, coincident with the "heavy bombardment" that scarred most bodies in the solar system early in its history.

Magnetosphere

Venus's magnetosphere is far too weak to protect it from the radiation of the solar wind. Its magnetic dipole moment cannot be more than 8 * 1017 N-m/T, or about 10−5 times that of the earth. Given its mass, it should have had a magnetic dipole moment at creation of 1.15 * 1024 N-m/T. Thus its decay time is very short (433 Julian years) and its half-life even shorter (300 Julian years). The conventional explanation is that the very slow rotation of Venus precludes any dynamo effect. Russell Humphreys, however, suggests that Venus simply has a small and relatively non-conductive core and was thus less able to maintain its magnetic field since creation as well as Earth has.

Satellites

Venus has no satellites today. But a number of planetary scientists now suggest that Venus once did have a satellite, but a double giant-impact event destroyed it and also set Venus to rotating retrograde rather than prograde. The projected sequence is as follows:[15]

The first impacting body was a body the size of Mars that struck Venus hard enough to remove some of its mass. This is similar to the giant-impact theory of the origin of the Moon of Earth. In Venus's case, the impacting body did not distort the gravity of Venus enough to allow escaping material to accrete into a moon, or a moon did form but then escaped the gravity of Venus entirely. The main difficulty with this theory is that such escape might have required tens of billions of years, longer than a reasonable age of the solar system or even of the universe.

The second impacting body stopped or reversed Venus's rotation and also so altered the gravity field of Venus that its moon spiraled inward and ultimately crashed onto Venus and added back to its mass.

Young Surface

Today, the surface of Venus is very young and has a relatively low number of craters in comparison to most other terrestrial bodies in the solar system including the moons of various planets, asteroids, Mars, Mercury, and even the Earth. This could possibly be explained in part by the very dense atmosphere of Venus which burns up most small objects that attempt to reach it's surface, creating meteors. The young surface can also be attributed in part to the high level of volcanic activity on Venus. Venus is one of a handful of notable solar system bodies including the Earth, Io and Triton which have various degrees of observable activity. On Venus, the combination of high levels of volcanism to constantly repave the surface of the planet combined with the dense atmosphere to protect the fresh volcanic flows provide for the young appearance of the planet.

Problems with uniformitarian theories

Scientist David Grinspoon assumes, without discernible warrant, that Venus once had liquid-water oceans that have since evaporated.[2] Their sole grounds for so stating is that if Earth formed with such vast surface oceans, so Venus should have. Yet the atmosphere has no more than 20 ppm of water today. Moreover, the extensive explorations of Venus by both orbiters and landers has revealed no geological evidence that Venus ever had an ocean. (The conventionale explanation is that the resurfacing of Venus alluded to above would have destroyed any such evidence.)

Venus rotates retrograde to its orbit around the Sun. Nearly every other Solar System body rotates prograde. This presents a serious theoretical problem for the nebular hypothesis of the origin of the solar system. The usual suggested solution is a giant impact, or a series of impacts, that somehow reversed Venus' direction. But such models must also explain why these impacts left Venus in an almost circular orbit and with an axis tilted less than three degrees from being perpendicular to its orbital plane.

Observation and exploration

Venus is one of the best-explored bodies in all the solar system. The first fruitful observation of Venus as a celestial body, rather than as a mythical entity, was by Galileo Galilei. He first documented the phases of Venus, something that requires a 75-meter telescope to observe. Other astronomers readily discerned that Venus had an atmosphere, though some incorrectly surmised that the solar day on Venus was comparable to that on Earth.

The United States and the Union of Soviet Socialist Republics both sent multiple rocket probes to Venus. The Soviet Union holds the present record for attempted missions (successful or failed) to Venus, including Project Sputnik and the highly prolific Project Venera. The first several Venera landers were crushed by the 92-bar atmosphere before they could even reach the surface. But eventually the Soviets did succeed in launching probes that descended to Venus's surface by parachute and transmitted data and even images.

The American Project Mariner achieved some success, mainly with orbiters that either traveled to Venus directly or (in the case of Mariner 10) made brief rendez-vous with Venus on the way to Mercury. The USA also launched the Pioneer Venus series of missions, including one orbiter and a total of five atmospheric descenders.[16]

In 1967, a NASA contractor developed plans for sending an Apollo-style rocket with a three-man crew to make rendez-vous with Venus in 1974 and also fly close to Mercury and the Sun. The mission was projected to last a year by carrying an extra life-support module, using the spent S-IVB stage as expanded living quarters, and mounting a solar cell array system on the S-IVB.[17]

Project Magellan achieved a major research breakthrough with the arrival of the Magellan orbiter at Venus on August 10, 1990. Over four and a half years, the craft successfully mapped 98% of the planet's surface and 95% of its gravity field. Magellan determined that the planet's geography is largely volcanic, with many lava domes, plains and channels.

Most of these novels and dramatic projects depended on the possibility, still real before the first remote explorations of Venus, that Venus was as hospitable to life and even to civilization as is the Earth. The data from these first explorations showed that Venus is a most inhospitable place for any form of life. Though one dramatist had suggested that Venus might harbor dinosaur-like creatures that thrived on sulfur compounds, no creature larger than a microbe could survive Venus's searing heat, nor does any model exist for the finding of a thermophile or similar extremophile in that environment.

Since these revelations, the genre of science fiction involving civilizations or easy human colonization of Venus died. Today most authors who speculate about Venus rely on ideas for terraforming, or transforming Venus into an environment able to sustain Earth-derived life.